This invention relates to an image transmission system and an image reproducing device and, in particular, to a transmission system for transmitting a video code compressed and encoded by the use of discrete cosine transform (DCT) as orthogonal transform and an image reproducing device for reproducing such an encoded image,
When a video signal is digitized and recorded in a recording medium such as a CD-ROM, a hard disk, and a magnetic tape, the video signal is generally compressed and encoded to be recorded because the amount of data is enormously large.
Proposals have been made of various compression encoding systems. Among those, use is frequently made of transform encoding by the use of orthogonal transform which efficiently utilizes correlation within a two-dimensional space of an image. Specifically, encoding systems based on the DCT as orthogonal transform are adopted in international standard encoding systems such as a color still image standard JPEG (Joint Photographic Expert Group) and a moving picture encoding standard for storage media (MPEG: Moving Pictures Expert Group).
In the MPEG, one sequence of a moving image is divided into groups of pictures (GOP), each group comprising a plurality of frames (pictures), to be subjected to encoding, as will later be described. The GOP comprises three types of pictures including an I picture which is an intraframe encoded image, a P picture which is an interframe encoded image predicted from a time sequentially preceding frame already encoded, and a B picture which is an interframe encoded picture predicted from time sequentially preceding and succeeding two frames.
A video code according to the MPEG can be reproduced by any reproducing unit based on the IPEG which is an international standard. However, processing carried out in the decoding unit and the IDCT unit imposes a heavy load upon a central processing unit (CPU). Therefore, high-speed reproduction can not be carried out unless the CPU operable at a high speed is used.
For example, when a JPEG- or MPEG-based video code reproducing device processes 15 frames per second, reproduction of a single frame must be carried out in about 66 milliseconds. It is assumed here that decoding of the Huffman code requires 30 ms, dequantization requires 10 ms, IDCT by the IDCT unit requires 20 ms, and display requires 20 ms. In this event, a total processing time of 80 ms is required. As a result, a delay of 14 ms is produced upon reproduction of an image of a single frame.
In order to reduce the load imposed upon the CPU, various proposals have been made in which the encoding system is partially modified so that high-speed reproduction can be carried out even with a low-speed CPU. For example, such proposals have been made in Japanese Patent Publications JP-A 241270/1990, JP-A 971/1992, JP-A 256452/1991, JP-A 357788/1992, and JP-A 299688/1992.
However, each of the conventional image reproducing devices and encoding systems mentioned in the above-referenced publications carries out image compression by partially modifying the encoding system and is not compatible with the international standard encoding systems. Therefore, a specific reproducing unit is required.
In the conventional image compression by reducing the amount of the compressed video codes, it is necessary to judge an optimum compression method for every block. Accordingly, preparation of the compressed codes requires a long time. In addition, the amount of the codes does not correspond to the processing ability of the image reproducing device. Accordingly, realtime reproduction is impossible by a reproducing device having a low processing speed. On the other hand, even with a high-speed reproducing device, no other than those codes having a predetermined picture quality can be reproduced because the amount of the compressed codes is determined.